During the past decade, there has been a notable increase in the industrial use of high-speed machining technology. During this time, machining centers have been developed that are capable of operating with spindle and slide speeds that are an order of magnitude higher than spindle and slide speeds that are available on older machining centers. While these new machining centers offer the possibility of much higher material removal rates, improved surface finish, and increased workpiece quality, use of these new machining centers also requires more technical expertise than conventional machining centers, especially in monitoring the cutting processes.
In machine tool cutting operations, process forces are created between the workpiece and tool, and results in deformation. Measuring the process forces has become important, specifically in microcomponents produced for biomedical, aerospace, electronics and mold production applications. Process forces may be measured to determine when a process is no longer proceeding as it should and correction is needed by identifying when process forces deviate outside the usual range in continuous or cyclically repetitive processes. However, the usefulness of the measured process forces is determined by the accuracy and functional reliability of the sensors employed for monitoring the process forces.
A basic element in determining process performance is the ability to sense the force that the tool is experiencing. Such force data has indicated that tools undergo three stages of wear: a break-in stage, a relatively long and mild abrasive wear regime, and a final stage comprising an abrupt transition to severe adhesive wear that results in catastrophic failure of the tool. Progressive wear on cutting edges during such wear stages is accompanied by changes in the forces that are generated by the cutting tool operating at programmed speed and feed rates. Ideally, a tool is removed from service when the cutting forces reach a level corresponding to an end of an abrasive wear state or useful life of a tool before catastrophic failure of the tool. However, in micro-milling applications, a tool may need to be replaced earlier in the process due to the precision needed in these cutting applications.
The use of microcomponents in many applications has caused more focus to be placed on the accuracy in cutting processes used to make these components. The pursuit of smaller and smaller components has led to the development of micro-milling processes that are characterized by high spindle speeds (up to 500,000 rpm and higher) and low cutting force magnitudes (<1 N or 0.22 bf). Due to these high spindle speeds, the tooth passing frequency, i.e., the fundamental cutting force frequency, may reach 16 kHz or higher for a two flute cutter, thereby making accurate measurement of these cutting forces even more important. One manner of measuring cutting forces in this application has been through the use of a piezoelectric dynamometer. High frequency bandwidth dynamometers have also been used to identify unstable cutting conditions, or chatter, in tools by examining a frequency spectrum of the cutting force signal. Significant content at frequencies other than the tooth passing frequency or its harmonics typically signifies chatter and can be used to select stable operating spindle speeds.
There are many operations that generate high frequency signals that may be measured. The ability to accurately measure these high frequency signals enables the process to be more precisely controlled, thereby increasing the operating efficiencies of these processes. One example of a process having a high frequency is a cutting operation wherein the process of a tool cutting into an object results in forces and vibrations. In such a process, the faster the operating speed of the tool, the higher the frequency of the forces and, in general, vibrations. However, the frequencies produced by tools operating at such high speeds may occur outside the effective range of prior art dynamometers.
Sharp tools typically have different cutting characteristics from dull or worn tools. In particular, wear lands develop on the cutting tool, which causes more of the cutting tool to come into contact with the workpiece during the cutting process. The increased contact area between the tool and workpiece causes more energy to be consumed by the cutting machine while making a cut because more energy is expended on non-productive work. Typically, increased forces necessary to operate a spindle in lathes, milling machines, etc. are indicative of an increase in non-productive work. In general, as the tool wears, the cutting force experienced by the tool increases.
Commercially-available piezoelectric dynamometers typically specify a bandwidth below the first natural frequency of the dynamometer structure, such as between 1 kHz and 3 kHz. Operation in this range enables the actual force to be determined by multiplying the time-domain dynamometer signal by a calibration constant. For operation outside this range, a force may be determined by multiplying the sensor signal (transformed to the frequency domain) by the inverted dynamometer frequency response function (FRF) to remove the influence of the dynamometer dynamics from the recorded signal. However, this approach is limited because the frequency response magnitude is generally very small at higher frequencies making it impossible to recover a valid force signal due to a poor signal-to-noise ratio. Additional difficulties encountered in measuring low cutting forces at high frequencies include: 1) a high resolution measurement device with a very large bandwidth is required, 2) a method is needed to excite the structure with a known input at all frequencies of interest to determine the FRF of the dynamometer system, and 3) a dynamometer system is needed that is capable of limiting the effect of forces in directions other than the direction of interest.
Thus, a need exists for an apparatus for accurately determining forces having a high frequency. A need also exists for an apparatus for determining force exerted in high-frequency applications. Finally, a need exists for an apparatus for determining force in one or more directions while limiting the effects of forces in other directions.